Conventional cordless window coverings generally include a headrail, a bottom rail, a covering material, a spring box, and a damper. The covering material is hung between the headrail and the bottom rail. The covering material is collected or expanded through ascending or descending the bottom rail. Upon expansion of the covering material, the spring box is actuated by descending the bottom rail, and energy is stored in the spring box. While the damper is also actuated by the descending bottom rail to slow down the descending speed to avoid dropping the covering material too fast due to gravity, and thus preventing the covering material from striking objects or individuals below the window covering. Conversely, upon collection of the covering material, the energy stored in the spring box drives the bottom rail to ascend and return to the initial position.
During the descending of the bottom rail, the damper is activated to enable a slower descending speed of the bottom rail and prevent the bottom rail dropping by gravity. However, while the bottom rail descends away from the headrail, the weight of the covering material loaded on the bottom rail decreases gradually, resulting in a very slow descending speed of the bottom rail. In some circumstances, the bottom rail may even stop descending and the covering material cannot be fully expanded. Therefore, conventional cordless window coverings need to be improved with a control of the output of the damper to enable the covering material to expand fully and smoothly during the descending process of the bottom rail.